Zinc oxide varistors are ceramic semiconductor devices based on zinc oxide. They have highly non-linear current/voltage characteristics, similar to back-to-back Zener diodes, but with much greater current and energy handling capabilities. Varistors are produced by a ceramic sintering process which gives rise to a structure consisting of conductive zinc oxide grains surrounded by electrically insulating barriers. These barriers are attributed to trap states at grain boundaries induced by additive elements such as bismuth, cobalt, praseodymium, manganese and so forth.
Fabrication of zinc oxide varistors has traditionally followed standard ceramic techniques. The zinc oxide and other constituents are mixed, by milling in a ball mill, and are then spray dried, for example. The mixed powder is dried and pressed to the desired shape, typically tablets or pellets. The resulting tablets or pellets are sintered at high temperature, typically 1,000.degree. to 1,400.degree. C. The sintered devices are then provided with electrodes, typically using a fired silver contact. The behavior of the device is not affected by the configuration of the electrodes or their basic composition. Leads are then attached by solder and the finished device may be encapsulated in a polymer material to meet specified mounting and performance requirements.